This invention relates to feedback loop control systems. More particularly, it relates to feedback loop control systems for controlling elements which have a variable gain and which produce an output signal proportional to the element gain.
In most feedback loop control systems, the basic problem to be solved is simply that of generating and feeding back an error correction signal whose parameters are essentially independent of any effects of gain by the element to be controlled. In such a situation the gain to be applied to the error correction signal remains at all times substantially constant, regardless of the output of the controlled element, since the controlled element has no impact upon the bandwidth of the feedback control loop. Conversely, in a conventional automatic gain control circuit the problem addressed is that of amplifying or attenuating a signal utilized in an open loop control system. In that situation the applied gain is utilized for stabilization of another parameter of operation of the controlled element which does not itself affect the applied gain. Heretofore, it has not been necessary to address the compounded problem of a second order stabilization in which it is necessary to stabilize the loop gain of the system stabilizing feedback loop, in which the controlled element itself causes variations in the loop gain and in which the operating parameters of the controlled element thus affect the bandwidth of the feedback control system. In such a compounded feedback control system it is necessary to stabilize the feedback loop gain to compensate for variations in the gain of the controlled element in order to suppress noise and stabilize the operating parameter of interest, such as output frequency or output signal level.
One such variable gain element for which feedback control is desirable is a laser, whose gain and thus light output intensity is directly related to the current input thereto. Since the gain of a laser thus varies with light output, the bandwidth of any conventional feedback loop control system used with the laser would also vary with the intensity of light output, due to the impact on loop gain caused by the variable gain of the laser. Thus, in the use of a conventional feedback control system with a laser having a modulated output intensity, the increased noise at lower output levels would render it increasingly difficult for the laser to follow accurately the intensity modulation commanded by the control loop. Similarly, the use of conventional feedback control loop with a variable frequency laser, such as a dye laser, would render it difficult to control and maintain accurately the frequency of the laser output whenever the intensity of the laser drops, reducing the bandwidth of the output signal. Further, stabilization of the output intensity variation in a pump laser used to pump such a dye laser would encounter similar difficulties where the variably noisy output signal is compared against a reference signal having little or no noise.
To illustrate the stabilization problem more specifically, the case of the tunable broadband dye laser is instructive, since it is capable of providing output radiation at wavelengths ranging over a comparatively large segment of the visible spectrum but experiences substantial gain variations when significant changes are made in its output wavelength.
With the development of this dye laser having the ability to vary its output frequency a new dimension in precision spectroscopy was provided, allowing illuminating subjects of interest with the output radiation from such a laser at any of a broad range of output frequencies. For such purposes it is often necessary that the output radiation of the laser be confined to a very narrow range of frequencies and that such output be stabilized to maintain the output frequency substantially constant over a relatively long time period of several minutes or even hours or more. Tuning of such a laser system conveniently may be achieved by a variable optical frequency discriminator such as a high finesse optical cavity receiving a portion of the laser output radiation and forming a part of a servo loop controlling one or more frequency adjusting element within the laser cavity. One approach to this control method is described in a co-pending patent application to Berg and Wise, entitled "Method and Apparatus for Providing a Calibrated Scan for a Scanning Laser," which is assigned to the assignee of this application. The frequency stabilization technique used with such apparatus is that of frequency offset locking using the side of a transmission fringe of the optical frequency discriminator or reference cavity, with the zero for the error signal located approximately half-way up the fringe. This technique is described in "Frequency Stablilization of a CW Dye Laser" by Barger, Soren and Hall, 22 Applied Physics Letters No. 11, pp. 573-75 (1973). With such an approach, using a fast differencing technique employing a separate laser output intensity reference channel, a frequency error signal may be obtained whose variations are substantially independent of the minor fluctuations in laser output intensity in the spectral region of choice. However, although the frequency error signal variations are substantially independent of laser intensity fluctuations on a proportional basis, the absolute magnitude of such signals inherently varies with the output intensity, thus requiring an offsetting adjustment in some frequency stabilization apparatus for major changes in level of the laser output radiation intensity, such as occur when the spectral range of operation of the laser is changed significantly. In prior art laser systems such adjustments are generally made by manual adjustment of a potentiometer.
While such a manual adjustment technique is quite suitable for a tunable broadband laser system in which the spectral region of interest is infrequently changed, the necessity for checking and manually resetting the feedback loop gain applied to the frequency error signal is most inconvenient and disadvantageous in a laser system which may quickly be tuned over a very broad spectral range, such as the 400 nanometer (nm) to 800 nanometer (nm) range of a tunable dye laser. When tuning over such a broad range, the gain of the laser, which is part of overall loop gain, may vary dramatically, even as much as 100:1. Additionally, conventional automatic gain control (AGC) techniques in which the output signal of a circuit, in this case the frequency error signal, is maintained at a generally constant level are inapplicable to this problem. This is so because the signal of interest is an error feedback signal which is ultimately driven to zero instead of the conventional constant level output signal, so that attempted maintenance of this error feedback signal at a constant level as the frequency error is driven to zero would result in the command of feedback loop again to increase, approaching infinity, quickly leading to undesired oscillation.